Congestion in DataNetworks      By       shaik shabaz ahmed       Regd no. 309175710104
What Is Congestion?->Congestion occurs when the number of packetsbeing transmitted through the network approachesthe packe...
Queues at a Node
Effects of Congestion->Packets arriving are stored at input buffers->Routing decision made->Packet moves to output buffer-...
Interaction of Queues
IdealNetworkUtilization
Practical Performance->Ideal assumes infinite buffers and no overhead->Buffers are finite->Overheads occur in exchanging c...
Effects ofCongestion -No Control
Mechanisms forCongestion Control
Backpressure->If node becomes congested it can slow down or halt flowof packets from other nodes->May mean that other node...
Choke Packet->Control packet  --Generated at congested node  --Sent to source node  --e.g. ICMP source quench     ->From r...
Implicit Congestion Signaling->Transmission delay may increase withcongestion->Packet may be discarded->Source can detect ...
Explicit Congestion Signaling->Network alerts end systems of increasingcongestion->End systems take steps to reduce offere...
Categories of Explicit Signaling->Binary  --A bit set in a packet indicates congestion->Credit based  --Indicates how many...
Traffic Management->Fairness->Quality of service  --May want different treatment for different connections->Reservations  ...
Congestion Control in PacketSwitched Networks->Send control packet to some or all source nodes  --Requires additional traf...
Frame RelayCongestion Control->Minimize discards->Maintain agreed QoS->Minimize probability of one end user monoply->Simpl...
Techniques->Discard strategy->Congestion avoidance->Explicit signaling->Congestion recovery->Implicit signaling mechanism
Traffic Rate Management->Must discard frames to cope with congestion  --Arbitrarily, no regard for source  --No reward for...
Operation of CIR
RelationshipAmongCongestionParameters
Explicit Signaling->Network alerts end systems of growingcongestion->Backward explicit congestion notification->Forward ex...
ATM Traffic Management->High speed, small cell size, limited overhead bits->Still evolving->Requirements  --Majority of tr...
Latency/Speed Effects->ATM 150Mbps->~2.8x10-6 seconds to insert single cell->Time to traverse network depends on propagati...
Cell Delay Variation->For ATM voice/video, data is a stream of cells->Delay across network must be short->Rate of delivery...
Cell Delay VariationAt The UNI->Application produces data at fixed rate->Processing at three layers of ATM causes delay  -...
Timings Considered->Cell insertion time->Round trip propagation time->Connection duration->Long term->Determine whether a ...
Traffic Management andCongestion Control Techniques->Resource management using virtual paths->Connection admission control...
GFR Traffic Management->Guaranteed frame rate is as simple as UBR fromend system viewpoint->Places modest requirements on ...
Frame Recognition->GFR recognizes frames as well as cells->When congested, network discards whole framerather than individ...
ANY QUERIES       THANK YOU
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Congestionin datanetworks

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Congestionin datanetworks

  1. 1. Congestion in DataNetworks By shaik shabaz ahmed Regd no. 309175710104
  2. 2. What Is Congestion?->Congestion occurs when the number of packetsbeing transmitted through the network approachesthe packet handling capacity of the network->Congestion control aims to keep number ofpackets below level at which performance falls offdramatically->Data network is a network of queues->Generally 80% utilization is critical->Finite queues mean data may be lost
  3. 3. Queues at a Node
  4. 4. Effects of Congestion->Packets arriving are stored at input buffers->Routing decision made->Packet moves to output buffer->Packets queued for output transmitted as fast aspossible --Statistical time division multiplexing->If packets arrive to fast to be routed, or to be output,buffers will fill->Can discard packets->Can use flow control —Can propagate congestion through network
  5. 5. Interaction of Queues
  6. 6. IdealNetworkUtilization
  7. 7. Practical Performance->Ideal assumes infinite buffers and no overhead->Buffers are finite->Overheads occur in exchanging congestioncontrol messages
  8. 8. Effects ofCongestion -No Control
  9. 9. Mechanisms forCongestion Control
  10. 10. Backpressure->If node becomes congested it can slow down or halt flowof packets from other nodes->May mean that other nodes have to apply control onincoming packet rates->Propagates back to source->Can restrict to logical connections generating most traffic->Used in connection oriented that allow hop by hopcongestion control (e.g. X.25)->Not used in ATM nor frame relay->Only recently developed for IP
  11. 11. Choke Packet->Control packet --Generated at congested node --Sent to source node --e.g. ICMP source quench ->From router or destination ->Source cuts back until no more source quench message ->Sent for every discarded packet, or anticipated->Rather crude mechanism
  12. 12. Implicit Congestion Signaling->Transmission delay may increase withcongestion->Packet may be discarded->Source can detect these as implicit indicationsof congestion->Useful on connectionless (datagram) networks --e.g. IP based ->(TCP includes congestion and flow control - see chapter 17)->Used in frame relay LAPF
  13. 13. Explicit Congestion Signaling->Network alerts end systems of increasingcongestion->End systems take steps to reduce offered loadBackwards --Congestion avoidance in opposite direction to packet required->Forwards --Congestion avoidance in same direction as packet required
  14. 14. Categories of Explicit Signaling->Binary --A bit set in a packet indicates congestion->Credit based --Indicates how many packets source may send --Common for end to end flow control->Rate based --Supply explicit data rate limit --e.g. ATM
  15. 15. Traffic Management->Fairness->Quality of service --May want different treatment for different connections->Reservations --e.g. ATM --Traffic contract between user and network
  16. 16. Congestion Control in PacketSwitched Networks->Send control packet to some or all source nodes --Requires additional traffic during congestion->Rely on routing information --May react too quickly->End to end probe packets --Adds to overhead->Add congestion info to packets as they crossnodes --Either backwards or forwards
  17. 17. Frame RelayCongestion Control->Minimize discards->Maintain agreed QoS->Minimize probability of one end user monoply->Simple to implement --Little overhead on network or user->Create minimal additional traffic->Distribute resources fairly->Limit spread of congestion->Operate effectively regardless of traffic flow->Minimum impact on other systems->Minimize variance in QoS
  18. 18. Techniques->Discard strategy->Congestion avoidance->Explicit signaling->Congestion recovery->Implicit signaling mechanism
  19. 19. Traffic Rate Management->Must discard frames to cope with congestion --Arbitrarily, no regard for source --No reward for restraint so end systems transmit as fast as possible --Committed information rate (CIR) ->Data in excess of this liable to discard ->Not guaranteed ->Aggregate CIR should not exceed physical data rate->Committed burst size->Excess burst size
  20. 20. Operation of CIR
  21. 21. RelationshipAmongCongestionParameters
  22. 22. Explicit Signaling->Network alerts end systems of growingcongestion->Backward explicit congestion notification->Forward explicit congestion notification->Frame handler monitors its queues->May notify some or all logical connections->User response --Reduce rate
  23. 23. ATM Traffic Management->High speed, small cell size, limited overhead bits->Still evolving->Requirements --Majority of traffic not amenable to flow control --Feedback slow due to reduced transmission time compared with propagation delay --Wide range of application demands --Different traffic patterns --Different network services --High speed switching and transmission increases volatility
  24. 24. Latency/Speed Effects->ATM 150Mbps->~2.8x10-6 seconds to insert single cell->Time to traverse network depends on propagation delay,switching delay->Assume propagation at two-thirds speed of light->If source and destination on opposite sides of USA,propagation time ~ 48x10-3 seconds->Given implicit congestion control, by the time droppedcell notification has reached source, 7.2x106 bits have beentransmitted->So, this is not a good strategy for ATM
  25. 25. Cell Delay Variation->For ATM voice/video, data is a stream of cells->Delay across network must be short->Rate of delivery must be constant->There will always be some variation in transit->Delay cell delivery to application so thatconstant bit rate can be maintained to application
  26. 26. Cell Delay VariationAt The UNI->Application produces data at fixed rate->Processing at three layers of ATM causes delay -->Interleaving cells from different connections operation and maintenance cell interleaving -->If using synchronous digital hierarchy frames, these are inserted at physical layer -->Can not predict these delays
  27. 27. Timings Considered->Cell insertion time->Round trip propagation time->Connection duration->Long term->Determine whether a given new connection canbe accommodated->Agree performance parameters with subscriber
  28. 28. Traffic Management andCongestion Control Techniques->Resource management using virtual paths->Connection admission control->Usage parameter control->Selective cell discard->Traffic shaping
  29. 29. GFR Traffic Management->Guaranteed frame rate is as simple as UBR fromend system viewpoint->Places modest requirements on ATM networkelements->End system does no policing or shaping of traffic->May transmit at line rate of ATM adaptor->No guarantee of delivery --Higher layer (e.g. TCP) must do congestion control->User can reserve capacity for each VC --Assures application may transmit at minimum rate without losses --If no congestion, higher rates maybe used
  30. 30. Frame Recognition->GFR recognizes frames as well as cells->When congested, network discards whole framerather than individual cells->All cells of a frame have same CLP bit setting->CLP=1 AAL5 frames are lower priority --Best efforts->CLP=0 frames minimum guaranteed capacity
  31. 31. ANY QUERIES THANK YOU

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